A patient with von Willebrand's disease was transfused with a small amount of fresh frozen plasma. Factor VIII coagulant activity reached a peak at the 6th hour after transfusion whereas the factor VIII-related antigen peaked at the 3rd hour and started to decline rapidly. The ristocetin-Willebrand factor rose immediately after the transfusion to a level which would have been predicted from the amount of this activity transfused and remained at levels slightly lower than this for the remainder of the study. Transfusion of the patient's own plasma obtained at the height of the factor VIII response produced no increase in any of the activities measured. Factor VIII stimulated by transfusion is more labile in vitro than normal Factor VIII.
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